let width: number | undefined | null;
width = getWidth()
if (width === undefined) {

}
// ?相当于string |undefined
function getPeople(name?: string) {
    return name || ''
}
getPeople('zhangsan')
getPeople(undefined)
function getWidth(): number | undefined | null {
    if (Math.random() > 0.3) {
        return undefined
    }
    else if (Math.random() > 0.6) {
        return null
    }
}
//any
let school: any
school = null
school = null
school = 'hello'
school = 3